Effective resuscitation of burn injuries is critical for lowering both the mortality and morbidity rates of burn patients. Both treatment and rehabilitation of burn injuries requires a large economic investment by hospitals in terms of cost and long term intensive care requirements for patients with severe and/or large percentage body burns. It is not uncommon that a normal size adult will receive over 30 liters of fluid while having urinary output (or urine output) totaling less than 2 liters, which results in a gain of about 60 pounds from the fluid retention in the body resulting from, for example, capillary leakage in response to the injury.
Each year approximately 45,000 adults and 15,000 children require hospitalization due to burn injury with 5,000 dying due to the severity or complications resulting from their injuries. For the military population, injury patterns due to current conflicts may include both traumatic and burn injuries that necessitate immediate treatment. Furthermore, recent studies have shown that over resuscitation of burn injury is not uncommon, resulting in significant iatrogenic complications.
Critical to survival are the initial 48 hours of post-burn resuscitation; however, this time period is extended in situations where the patient takes a long time for care and eventual transport such as occurs when bringing burned soldiers from the Iraq theater to the U.S. Army Institute of Surgical Research (USAISR) burn unit at Fort Sam Houston, Tex. During this phase, patients require prompt initiation of fluid therapy, and around-the-clock care by experienced burn surgeons and intensivists. However, advanced burn care expertise is not found in most hospitals, and the care outside of burn centers can lead to increase morbidity from infusing too much fluid. This limitation includes receiving centers, whether they are civilian emergency rooms, forward military facilities or ad hoc medical facilities for mass casualty. Because acute burn care is particularly labor intensive, burn injuries sustained in mass casualties can quickly overwhelm even the best hospitals and burn centers. Clearly, there is a need to reduce the workload of advanced burn centers and to impart burn expertise to less specialized medical facilities.
The pathophysiologic response to large thermal injuries (≧30% of total body surface area [TBSA]) is characterized by substantial plasma extravasation and general edema formation, leading to intravascular volume depletion and burn shock. Delayed or inadequate fluid resuscitation is associated with increased morbidity and mortality. Initial treatment currently consists of isotonic crystalloid infusion based on a regimen that is directed towards volume replenishment to obtain cardiovascular stabilization and maintain adequate renal function. However, such treatment is only partially effective due to an array of circulatory mediators and sustained fluid extravasations into the extravascular space.
a. Current Resuscitation Regimens
Defining the best solutions, infusion rates, and volume requirements for resuscitation of burn injury has been an ongoing research focus for the last 100 years. Several formulas have been developed to guide the care provider with a predicted infusion volume for the first 24 hours and with a specific initial infusion rate based on the size of the burn injury and patient weight. Infusion rates are adjusted hourly, based on the urinary output (UO) of the patient during the last measured period. The most common contemporary infusion formulas are the Brooke formula (2 ml/kg per % TBSA for 24 hours) and the Parkland formula (4 ml/kg per % TBSA for 24 hours). Fluids are periodically adjusted to maintain an adequate urinary output, within a predetermined target range. The rationale for using urinary output as the target endpoint to adjust fluid therapy is that if urinary output is normal then glomerular filtration rate, renal blood flow, and cardiac output are likely to be adequate. Target values are based on ranges determined by age (adult or pediatric), patient weight, and sometimes other factors that contribute to normal renal output. Adult target values are 0.5-1.0 ml/kg per hour or 30-50 ml/hr. Pediatric patients often require larger volumes due to greater insensible losses, and have a formula with a higher target urinary output of 1.0-2.0 ml/kg per hr. Maintaining urinary output targets is expected to normalize renal function, while avoiding excess or inadequate fluid infusion that may lead to an increase in complications or mortality. But recent reviews have suggested that this approach frequently leads to severe over-resuscitation, with many burn units administering mean volumes larger than the Parkland recommendation.
The current standard of care for patients receiving burn resuscitation is paper charts that include a flow sheet similar to that illustrated in FIG. 1A. In some situations where electronic charting is used, the monitors will provide data to the electronic charting system as illustrated in FIG. 1B, which is only a slight improvement over the flow sheet since there is no analysis of the data.
To evaluate contemporary methods of burn resuscitation, a meta-analysis of the last 26 years of burn resuscitation was conducted. A search of Medline for all clinical burn studies in which fluid resuscitation was guided by the Brooke or Parkland formula with adjustment in infusion rates to restore and maintain target urinary output was done. Data from 31 studies, which included 40 groups and 1,498 patients was extracted. FIGS. 2A and 2B show the total 24-hr volumes infused and the mean urinary outputs, respectively. Mean percentage of total body surface area (% TBSA) was 45±2% and mean fluid intakes were 5.1±1.3 mL/kg per % TBSA, with mean 24-hr urinary outputs of 1.1±0.4 mL/hr per kg. All studies reported mean volume administration exceeding the Brooke formula and 86% of studies reported mean values above the Parkland formula. In general, patients are resuscitated to achieve levels of urinary output that are at or above the high end of target level. However, most of the burn centers infused sufficient lactated Ringer's solution (LR) to induce mean 24-hour urinary outputs exceeding 1.0 mL/kg. The primary conclusions from the meta-analysis are: (1) total volumes infused typically exceed the Parkland formula and Advanced Burn Life Support (ABLS) guidelines, and (2) urinary outputs tend to be on the high side of ABLS guidelines.
The meta-analysis did not determine if burn centers are infusing more fluid than is optimal or if the Brooke and Parkland burn formulas specify inadequate volumes. A meta-analysis based on summary statistics of individual studies has limited power to determine relationships between fluid volumes and outcomes. Detailed individual patient data are needed to accurately determine the impact of fluid therapy on outcomes. Individual patient data is required to statistically correlate outcomes with total volumes infused and net volume retained (in minus out). Hourly data on infusion rates, urinary output and net volume (edema) is needed to fully define the relationships between volume therapy and urinary output in burn patients.
Reduced survival and more often increased morbidity are linked to sub-optimal resuscitation. But it is unknown how many patients are harmed by under- and over-resuscitation. From the meta-analysis, case reports, and clinical experience we know that individual burn experts resuscitate patients differently and that they usually produce clinical results deemed satisfactory. This may speak more to the physiological reserves of the patients and the ability of their kidneys to compensate for over-resuscitation than it does to our medical knowledge or expertise. A quip often used by intensivists is “the dumbest kidney knows more than the smartest intern.” Patients have effective compensatory mechanisms that can often compensate for a wide range of infused volumes. “Successful clinical results,” however, are not necessarily equivalent to optimal outcomes.
b. Fluid Creep
The need for large volume therapy for burn shock was identified in 1968 by Charles Baxter, who showed that successful resuscitation could be accomplished with a “Parkland formula” of 4-ml/kg per % TBSA of lactated Ringer's solution in the first 24 hours of care. Baxter C R, et al., Physiological response to crystalloid resuscitation of severe burns, Annals of the New York Academy of Sciences, 1968, vol. 150, pp. 874-894. Prior to that time, fluid therapy was largely performed with plasma and albumin solutions at lower volume totals. Subsequently, Pruitt et al. provided an alternate “Brooke formula” of 2-ml/kg per % TBSA. Pruitt BA Jr., Fluid and electrolyte replacement in the burned patient, Surg Clin N Am., 1978, vol. 48, pp. 1291-1312. The Advanced Burn Care Life Support (ABLS) guidelines established by the American Burn Association accepted these formulas and recommend a 2-4 mL/kg per % TBSA range of total fluid volumes for the first 24 hours, with the infusion rate adjusted to maintain a urinary output of between 0.5 mL/kg and 1.0 mL/kg per hr or about 30-50 ml/hr. American Burn Association, Advanced Burn Life Support Course (ABLS), Instructor's Manual, 2001. Nevertheless, burn centers routinely administer 25-50% more fluid than the Parkland formula recommends, and more than half the fluid is given within the first 8 hours. In clinical settings, physicians may accept high urinary outputs without decreasing infusion rates and more diligently increase infusion rates when urinary output is low. This viewpoint is supported by meta-analysis, which showed that mean urinary outputs and infused volumes were typically above ABLS guidelines.
The term “fluid creep” was first used by Pruitt to describe the increased volume of fluid that appears to be administered by burn centers in the first 24-48 post-burn hours. The morbidities associated with fluid overload include pulmonary edema and impaired gas exchange, abdominal compartment and intestinal ischemia syndromes, delayed wound healing, increased incidents of infection and sepsis, and multi-organ failure. Data supports benefits of reducing total infused volumes. Recently, perioperative and ICU trials of restricted fluid therapy showed improved outcomes. Brandstrup B et al., Effects of intravenous fluid restriction on postoperative complications: comparison of two perioperative fluid regimens: a randomized assessor-blinded multicenter trial, Annals of Surgery, 2003, vol. 238, pp. 641-648. Less net fluid accumulation has been associated with better outcomes in large burns treated with lactated Ringer's solution (LR). Cancio LC, Predicting Increased Fluid Requirements During the Resuscitation of Thermally Injured Patients, The Journal of Trauma, February 2004, vol. 56, No. 2, pp. 404-413. However, the correlation between increased survival and reduced fluid also reflects that the injury level correlates morbidity and mortality, and that more severely burned patients require more fluid.
Taken together the above findings suggest that optimal fluid resuscitation may be achieved by minimizing fluid accumulation, while maintaining adequate urinary output and cardiac output. However, the clinical consequences of more tightly controlled fluid therapy and urinary output to fall within established guidelines with less hourly variations are unknown.
c. Fluid Therapy Using Closed Loop Control
The concept of closed loop control is well established for industrial applications and its potential application to medicine has been extensively reviewed, although it has had limited utilization. Abbod MF, Survey on the use of smart and adaptive engineering systems in medicine, Artificial Intelligence in Medicine, 2002, vol. 26, pp. 179-209; Westenskow DR, Microprocessors in intensive care medicine, Medical Instrumentation, November-December 1980, vol. 14, no. 6, pp. 311-313. There have been clinical trials demonstrating effective closed loop control of nitroprusside infusion for postoperative blood pressure regulation in cardiac patients. Ying H, Fuzzy control of mean arterial pressure in postsurgical patients with sodium nitroprusside infusion, IEEE Transactions on Biomedical Engineering, 1992, vol. 39, pp. 1060-1070. Closed loop control of ventilators and delivery of anesthetics have evolved into commercially viable products. Brunner JX, Principles and history of closed-loop controlled ventilation, Respiratory Care Clinics of North America, 2001, vol. 7, pp. 341-362, vii; Wysocki M et al., Closed-loop ventilation: an emerging standard of care?, Critical Care Clinics, 2007, vol. 23, pp. 223-240, ix. Experimentally, closed loop fluid resuscitation has been used for treatment of hemorrhaged sheep using blood pressure, cardiac output, and tissue oxygen as endpoints. Chaisson NF et al., Near-Infrared Spectroscopy-Guided Closed-Loop Resuscitation of Hemorrhage, The Journal of Trauma, 2003, vol. 54, no. 5, pp. S182-S192; Rafie AD et al., Hypotensive resuscitation of multiple hemorrhages using crystalloid and colloids, Shock, 2004, vol. 22, pp. 262-269.
Bowman and Westenskow were the first to build a closed loop controller (using a proportional-integral-derivative (PID) algorithm) for fluid resuscitation of burn injury. Bowman et al., “A Microcomputer-Based Fluid Infusion System for the Resuscitation of Burn Patients,” IEEE Transactions on Biomedical Engineering, Vol. BM-28, No. 6, June 1981, pp. 475-479. In an era before personal computers were common, they built a specialized microprocessor for their controller. Both intake and urinary output were monitored with drop counters while a roller infusion pump was controlled with the PID algorithm. The PID algorithm was based on a mathematical model, which had been used to control resuscitation in a small number of dog experiments. They verified accurate monitoring of fluid in and urine out, but no control trials were performed in either animals or patients. Several decision trees and mathematical models of fluid balance after burn injury have been developed, but none has had significant clinical application. Bert J L et al., Microvascular exchange during burn injury: II. Formulation and validation of a mathematical model, Circulatory Shock, 1989, vol. 28, pp. 199-219; Bert J L et al., Microvascular Exchange During Burn Injury: IV. Fluid Resuscitation Model, Circulatory Shock, 1991, vol. 34, pp. 285-297; Roa L M et al., Analysis of burn injury by digital simulation, Burns Including Thermal Injuries, 1988, vol. 14, pp. 201-209.
An evaluation of individual hourly records of burn patients was done in order to define a current standard of care for burn resuscitation. Hourly fluid input and urinary output measurements from 20 adult burn patients were extracted from U.S. Army Institute of Surgical Research (USAISR) and University of Texas Medical Branch (UTMB) burn unit records. The data shown in FIG. 3 suggests great variability in urinary outputs before and after arrival at these two burn centers. Of 403 hourly in-hospital measurements in burn patients, 41% were below the ABLS target range of 1.0-2.0 mL/kg and 28% were above.
The principle conclusions from the analysis of these patients and of the literature meta-analysis are that mean urinary output above target levels predominated with infused volumes, even in advanced burn centers, exceeding ABLS guidelines. The tendency for clinicians to over-resuscitate burn patients may be responsible for many recognized complications such as abdominal compartment syndrome, extremity compartment syndrome, and airway edema requiring intubation, all of which are life- and/or limb-threatening. In particular, abdominal compartment syndrome was largely unheard before ten years ago, but is now a serious complication in many burn centers that results almost always in death.